staging: comedi: ni_labpc: move DMA init out of the common attach

Only the ISA versions of the LabPC boards support DMA. Move the DMA
initialization out of the labpc_common_attach() and put it in the
ISA specific labpc_attach() function.

Remove the 'dma_chan' parameter from labpc_common_attach().

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>